The invention relates to a device for moistening a material web moved in the transport direction, preferably for re-moistening a paper or textile web dried after printing by means of a spray device for spraying a water mist on the material web under the influence of an electrostatic field generated by an electrostatic charging device.
Devices for re-moistening of this type are known of themselves (EP-0 350 606 A2). They have proven themselves in practice. It is disadvantageous however that these known systems usually are difficult to retrofit in existing systems. Finally, only a moderate degree of re-moistening can be achieved with this known device.
The goal of the invention is to improve a device for moistening material web moved in a transport direction by means of a spray device for spraying a water fog onto the material web under an influence of an electrostatic field generated by a device for electrostatic charging so that a high degree of moistening can be achieved with small installation sizes.
This goal is achieved by providing a reversing roller for reversing the material web in the transport direction upstream of the spray device, the reversing roller having associated with it a device for electrostatic charging designed as a corona-charging electrode, the spray device having two water spray heads located on both sides of the material web.
It is important for the principle according to the invention that the material web was charged electrostatically in a transport direction before the spraying device (surprisingly more than in the area of the corona electrode in the vicinity outside of the reversing roller) and the material web thus charged influences the sprayed water mist as a result of the electrical field due to the charging, so that the water particles of the spray mist surprisingly are sucked up by the material web, in other words, the water aerosols with opposite charges relative to the paper penetrate the material while astonishingly the applied amount of water (about 2.5 g/m2) has not led to any formation of surface water on the material web. In addition, the moisture profile of the water spray heads has not formed itself on the material web, which may be due to the dominance of the high and uniform distribution of the field strength in the form of a homogeneous field. With this design according to the invention, an efficiency of more than 95% and sometimes even 98% has been reached. In contrast to the known devices, this also has the advantage that significantly less components are required which is directly related to the space requirements of such a device for re-moistening. The latter has the particular advantage of being able to retrofit existing pressure systems simply.